Entangled ZnO on Ultrathin Hollow Fibers for UV-Aided Pollutant Decomposition

Zinc oxide (ZnO), a widely used ultraviolet (UV) degrading substance, offers high selectivity for wastewater treatment, but the leaching of ZnO into water could cause secondary contamination. Using porous substrates to fix and load ZnO is a promising technical method to improve the water purification efficiency and recycling durability of ZnO. However, limited by the slow kinetics and shielding effects, it is challenging to use traditional techniques to introduce ZnO into the interior of a hollow structure. Here, inspired by an ancient dyeing procedure, we formed a unique single-molecule bio-interfacial entanglement as an absorption layer to capture the catalyst for ZnO electroless deposition (ELD) on the surface of natural ultrathin hollow-structured Kapok fibers. With curcumin serving as a linking bridge, ELD allowed the spontaneous formation of intensive ZnO nanocrystals on both the outer and inner walls. ZnO-kapok as the catalyst for ultraviolet photodecomposition of organic pollutants (methylene blue (MB) and phenol as model pollutants) delivered a decomposition efficiency of 80% and outstanding durability. Further modification of the ZnO-kapok catalyst by doping with reduced graphene oxide (rGO) showed an improvement in photodegradation performance of 90% degradation under 2-h irradiation with 21.85 W/dm2 light power. Moreover, to the best of our knowledge, this is the first report featuring ZnO loading on both the outer and inner walls of a fiber-structured hollow kapok material, which provides inspiration for immobilization of metallic oxides on hollow-structured materials for further applications in renewable catalysis, chemical engineering, and energy storage fields.


Dewaxing process
Kapok fibres were dewaxed by the enzyme (lipase). 2 g enzyme was dissolved into 100 mL phosphate buffer and 1 g kapok fibres were added into the solution stirred at 38 ℃ for 10 hours. Then, the pretreated fibres were rinsed 5 times in deionized water (DI water) and dried in an oven at 40 ℃ for 8 hours.

Preparation of graphene oxide in lab.
Graphene oxide (GO) was produced from graphite powder using a modified Hummers method. 54 ml of sulfuric acid and 6 ml of phosphoric acid were mixed and stirred for 10 mins. 0.45 g of graphite powder was added into the above mixture while stirring. Then 2.64 g of potassium permanganate (K 2 MnO 4 ) was added into the solution slowly and carefully and then stirred for 6 hours until the colour changed to dark green. To eliminate left KMnO4, 1.35 ml of hydrogen peroxide was dropped slowly into the solution with stirring for 10 mins. Then the mixture was left for cooling down. 20 ml of hydrochloric acid and 60 ml of DI water was added into the solution and centrifuged by Eppendorf Centrifuge 5430R at 8000 rpm for 10 mins. Then the supernatant was decanted away and the residuals were cleaned three times following above method. The GO solution was then dried in oven at 90 ℃ for 24 hours to get GO powder for later application. 1

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In the experimental section, all solvents and reagents were of analytic grade and were purchased from Sigma-Aldrich (Dorset, UK) and used without further purification. Sample mixtures were transferred into 1.3mm outer diameter (0.8mm inner diameter) silica tubes and then inserted into a 4mm outer diameter (3mm inner diameter) quartz EPR tube. The position of the sample in the resonator was optimised from measurements on a TEMPO standard solution.
Uniform UV irradiation at 365 nm was achieved using a Thorlabs M365L3 mounted LEDREF1 collimated using a Thorlabs SM2F32-A adjustable collimation adaptor. A 5x5mm offcut of the woven fabric was submerged and placed flat down into the bottom of glass vial, aliquots of the surrounding DMPO mixture were taken at 15 minutes and 30 minutes and transferred immediately into the capillary tubes.
X-band (9.4GHz) continuous wave EPR measurements were carried out on a Bruker EMXmicro EPR spectrometer equipped with a Bruker ER4122-SHQ resonator. Spectrometer settings were: microwave power 23dB (1.1 mW), modulation amplitude 0.5 G, sweep time 60 s receiver gain 30dB with an average microwave frequency of 9.86GHz.

Results and discussion
Characterization of kapok fibres samples. After curcumin modification, the palladium (Pd) ions might be chelated with the phenol groups of curcumin for dense ZnO electroless deposition. The palladium peak in the EDX spectra in Figure S1 indicates the presence of Pd on the kapok surface and 0.105 wt% Pd is measured.

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Also, it is apparent from the EDX images that palladium was distributed throughout the fibre area. Thus, the immobilization of catalysts on the fibres for ELD is ready. To prove the penetrability of UV light, the UV transmittance test was conducted. The UV transmittance spectra of pristine kapok (Figure S1g) have shown a poor UV blocking performance. In this case, the UV light, not utilized by the ZnO nanomaterials on the outer walls of fibre would have a chance to penetrate the fibre structure and be utilized by the ZnO nanocrystals loaded S-5 on the inner fibre walls. In addition, from the SEM images, it can be observed that kapok fibres have ultra-thin fibre walls (around 1 um). Therefore, these CKZ samples would make the most of the UV irradiation to enhance the photodegradation efficiency during the process.  The deposition of ZnO nanocrystals throughout the hollow fibres can prevent the general agglomeration and provide a highet specific surface area with activation sites. Compared with the raw kapok fibre, CKZ-120 min showed a type IV N 2 adsorption/desorption isotherm with the H3 adsorption hysteresis loops. 2 The predominantly mesopores structures of CKZ-120 min can be deduced from the abrupt adsorption at the high relative pressure area. The Brunauer-Emmett-Teller (BET) surface area of pristine kapok fibre, CKZ-120 min and CKZ-120 min after 5 cycles were 0.355, 5.712, and 1.254 m 2 /g, respectively, indicating the improvement on the specific surface area with the uniform nanocrystal layers on fibre substrate. The welldistributed nanostructured ZnO has provided a high specific surface area with many activation sites for the degradation process, which can lead to a distinctive improvement of UV degrading ability after the ELD process. However, as shown in Figure S4, the main pore size distribution of CKZ-120 min was between 10 to 80 nm while that of CKZ-120 min after 5 cycles was between 10 to 30 nm. The results suggested that ZnO nanocrystals may suffer some disassembly process due to corrosion after UV irradiation with a lower specific surface area and smaller pore sizes, compared with ZnO before UV degradation. Therefore, the electroless deposition of ZnO nanocrystals can enhance the specific area of natural fibres with plenty of active sites but may suffer some damage to the crystal structures after degradation.
S-8   UV degradation process of ZnO-kapok fibre-supported UV degradation catalysts for phenol is shown in Figure S8 after 5-hour UV light irradiation, which indicates that the UV degradation catalysts are not only able to degrade MB, but other organic pollutants.
S-12 Figure S9. A plausible degradation pathway of methylene blue Based on the previously reported photodegradation mechanism of ZnO nanomaterials towards methylene blue, a possible degradation pathway is presented in Figure S9. [3][4][5] S-13 ZnO powder under 5-hour darkness were around 17%, 20% and 17 %, respectively in Figure   S10. Furthermore, compared with the 3 control groups after 5 hours of UV irradiation, the degradation rates were 18%, 19% and 80 % respectively, which illustrated the photodegradation ability of the ZnO powders. Also, the absorption rates of the 3 control groups in a dark environment have suggested that the raw fibres can only offer limited absorption ability. In addition to this, the absorption rates of CKZs after 30 min dark absorptionequilibration experiment were around 25% shown in Figure 3d, which can be due to the surface modification of ZnO to improve the absorption ability. Hence, to achieve excellent water purifying results, both absorption and photodegradation ability are required. As kapok fibres can provide both the high surface area and excellent absorption ability, the fibre-based ZnO hybrid structure is a promising material for water treatment.
S-15   Figure S13 displays the UV degradation performance of MB after 5 reusing cycles by CKZ UV degradation catalysts after 5-hour UV irradiation, respectively. The TGA curves of CKZ-120 min before and after washing cycles showed the typical curve structure of fibre loaded with ZnO materials, suggesting the good durability of ZnO-loaded composite.
Video S1. Contact angle test of dewaxed kapok fibres modified with curcumin (30 s).